1. Field of the Invention
The invention relates generally to a method for monitoring a thermal coupling, and in particular, a thermal coupling between a measuring cell and a thermostatted element of an analyzer.
2. Description of Related Art
It is known that sensor elements of many measuring devices and analyzers have temperature-dependent signal properties. Depending on the particular type of sensor used, this temperature dependence is due to thermal influence on chemical processes, their equilibrium and/or their kinetics, or especially in the case of electrochemical sensors, is due to changes in the chemical-physical characteristics.
Such sensors are often used in medical analyzer systems for determining partial gas pressure, pH-value, or ion and metabolite concentrations in body fluids. In particular, such sensors are used in blood gas analyzers, used in medical diagnostics.
While the temperature coefficients of the sensors can be determined fairly easily by suitable calibration measurements, a problem arises in the measuring of blood gases and pH, in so far as the measured variables (pO2, pCO2, and pH) are temperature-dependent, and the temperature coefficients of the sample needed for computational correction are not known with sufficient precision. If the measured values obtained for a blood sample, for instance at room temperature, are to be computationally corrected to obtain values at body temperature (37° C.), the results will be imprecise.
In order to avoid the above mentioned temperature dependencies it is known to use sensors under controlled temperature conditions, i.e., in thermostats. When the measuring cells are to be exchanged after a certain period of use, the measuring cell and the thermostat, which is a fixed component of the analyzer, should be easily separable.
In general, the measuring cells are operated in a thermostatted chamber of the analyzer, which is kept at a constant temperature and is usually made of synthetic materials.
In order to optimally simulate the situation prevailing within the body of the patient, measurements are carried out at a sample temperature of 37° C. Even if only a short span of time elapses between the taking of the sample and the measurement, the blood sample will have cooled off significantly and usually will have to be rapidly reheated to body temperature in the measuring cell inside the analyzer.
For rapid and reproducible thermostatting it is essential to heat not only the media introduced into the measuring cell, such as calibrating media, control media, or sample fluids, to the required operating temperature in a fast and reproducible way, but also the sensors contained in the measuring cell.
A sensor device for measuring pH and blood gas parameters (pCO2, and pO2) described in U.S. Pat. No. 5,046,496 has the individual electrodes applied on a rectangular carrier plate of non-conductive ceramics by means of thick-film technology. The carrier plate with the measuring electrodes is glued into the housing of a flow-through cell. A temperature sensor and a heating element are also provided on the carrier plate in order to attain and control the temperature required for measurement.
A portable diagnosis system, in which the heating element is also directly integrated in the sensor chip with its individual electrodes, is known from U.S. Pat. No. 6,890,757 B2. In that system, the sensor chip is being monitored in a contact-free manner by an IR sensor for temperature measurement.
Finally, US Patent Application Publication No. 2003/0057108 A1 discloses a method for fast hydration and heating of chemical, electrochemical, and biochemical sensors. In that method the sensor cartridge consists of a lower part made of plastic, in which the sensors are provided, and of a metal cover plate, which can be used for heat transfer into the sensor cartridge. For this purpose the cover plate is in contact with suitable heating or cooling elements, for instance a Peltier element.
The additional expense incurred by integrating a heating element and a temperature measuring element into the measuring cell is a disadvantage of the above mentioned solutions.
An analyzer device with a thermostatted measuring cell having electrochemical electrodes is described in U.S. Pat. No. 7,491,175. In that analyzer device the measuring cell is thermostatted by Peltier elements, a flat, thermally conductive distributor element being placed between the Peltier elements and the wall of the measuring cell. This kind of thermostatting is equivalent to an air-bath on account of the unavoidable air gap, heat transfer mainly being limited by the thickness of the polymer material of low thermal conductivity surrounding the electrochemical sensor, and the remaining air gap against the thermostatted surface.
To improve heat transfer to the measuring cell, US Patent Application Publication No. 2006/0140822 disclosed a thermally conductive, elastic or plastic layer, which adheres at least in the contact area to a wall of the measuring cell or to the thermostatted supporting surface of the analyzer, and which can be removed without residue from the thermostatted supporting surface or the measuring cell wall, when the measuring cell is exchanged. US 2006/0140822 further proposed that the wall of the measuring cell, which carries one or more sensor elements on the inside facing the measuring channel, be made of a thermally conductive metal or metal alloy, at least in the contact area with the thermostatted supporting surface of the analyzer.
By such arrangements (thermally conductive layer or metal wall of the measuring cell), which can also be combined, heat transfer resistance between the heat source, the thermostatted supporting surface of the analyzer, and the sensor- or sample-plane is substantially minimized.
If the measuring cell is damaged or if the contact areas are considerably soiled, contact with the thermostatted surface is achieved only at a few points and in an irreproducible way, such that the quality of thermal coupling is degraded while the malfunction escapes quick detection.
As a consequence of insufficient thermal coupling temperature adaptation between sensor and sample is delayed. It will thus take longer to perform measurement at the desired target temperature, or the measuring is carried out prematurely, prior to reaching proper measuring temperature of 37° C., for instance.
Accordingly, the inventors have identified a need in the art to provide a method for checking the thermal coupling of a measuring cell inserted in an analyzer, which will avoid the use of a heating element and a temperature sensor in the measuring cell and permit rapid and reliable detection of any deterioration of thermal coupling when the measuring cell is exchanged, malfunctions or is in normal use.